1. Field of the Invention
The present invention relates to a high impedance circuit used as, for example, a signal input circuit, and particularly relates to a high impedance circuit and a high gain amplifier which are suitably used as a signal input circuit for a high frequency signal, such as a radio receiver, a television receiving set, a satellite broadcasting receiver, a video recorder and a mobile communication device.
2. Description of the Related Art
It is necessary that a signal input circuit has a high impedance. In addition, since a high speed operation is required to a signal input circuit used for a high frequency signal such as a signal for a radio receiver, a video signal for a television receiving set, a high frequency signal for a satellite broadcasting receiver, a video signal for a video recorder and an RF signal for a mobile communication device, normally such circuit is formed as a high impedance circuit having a differential-pair type circuit.
As a background art of the high impedance circuit of-the present invention, an example of a differential-pair type high impedance circuit having a differential-pair circuit which is suitable to a signal input circuit for a high frequency signal will be explained below with reference to FIG. 1.
FIG. 1 is a circuit diagram of a differential-pair type high impedance circuit as a background art of the high impedance circuit of the present invention.
This differential-pair type high impedance circuit comprises npn-type transistors Q11 and Q12 composing a differential-pair type circuit, both of the emitters are commonly connected and grounded to the ground potential GND via a resistance element R10. Bases of the npn-type transistors Q11 and Q12 are commonly connected and further connected to resistance elements R11 and R12. The high impedance circuit further comprises an npn-type transistor Q13 whose base is connected to the collector of the npn-type transistor Q11 and the emitter is connected to the base of the npn-type transistor Q11 via a current source circuit I11 and the resistance element R11, and an npn-type transist or Q14 whose base is connected to the collector of the npn-type transistor Q12 and the emitter is connected to the base of the npn-type transistor Q12 via the current source circuit I11 and the resistance element R12.
The high impedance circuit further comprises pnp-type transistors Q21 and Q22 connected to the collectors of the npn-type transistors Q11 and Q12, and resistance elements R21 and R22 connected between the transistors Q21 and Q22 and a first potential power source line Vcc in order that the differential-pair type circuit becomes high impedance. The base of the transistor Q21 and the base of the transistor Q22 are commonly connected. These pnp-type transistors Q21 and Q22 and the resistance elements R21 and R22 function as a direct current (DC) power source for the differential-pair type circuit.
The high impedance circuit is further provided with pnp-type transistors Q23 and Q24, resistance elements R23 and R24 connected to the emitters of these pnp-type transistors Q23 and Q24, and a current source circuit I12 for these circuits.
In this high impedance circuit, an input impedance Zin looked from the npn-type transistor Q13 is very high.
In the high impedance circuit shown in FIG. 1, the high impedance components depend on collector impedance of the transistors Q11 and Q12, that is, the impedance by circuits such as the pnp-type transistors Q21 and Q22. Accordingly, a bias is needed for providing direct current (DC) power sources at upper and under portions of the differential-pair type circuit in the high impedance circuit. Due to the bias, there arises a scale reduction of a dynamic range of more than about 1V, on the voltage of the first potential power source Vcc and the second potential power source GND. As a result, this high impedance circuit is not suitable for a low voltage operation.
Particularly, in a recent high impedance circuit for a high frequency signal, a wide dynamic range and operation at further lower voltage are required, therefore, the above disadvantage may be become large on a high impedance circuit for a high frequency operation.
Also, since the npn-type transistors Q11 to Q14 are used together with the pnp-type transistors Q21 to Q24 having an opposite polarity in the high impedance circuit in FIG. 1, it becomes complicated when manufacturing one integrated circuit (IC) accommodating these circuits.